The enzyme, O-GlcNAc transferase (OGT) that modifies nucleocytoplasmic proteins with O-GlcNAc has recently been shown to be part of the co-repressor complex turning off the transcription of genes. OGT associates with mSinSA and, therefore, is involved with genes that bind nuclear hormone receptors when the hormones, like estrogen, are not present. By modifying transcription activators, OGT cooperates with histone deacetylase to shut off transcription at sites where its scaffold, mSin3A, is present. Bound to OGT in a saturable fashion is NCOAT (nuclear cytoplasmic O-GlcNAcase and acetyltransferase), a bifunctional enzyme with O-GlcNAcase and histone acetyltransferase activities. NCOAT, by residing in co-repression complexes, allows the gene to be turned on again when hormone becomes present by reversing the actions of OGT and histone deacetylase. An NCOAT peptide that interacts with OGT, but has no enzymatic activities, blocks the ability of estrogen to induce estrogen-dependent genes or mammary development in transgenic mice. Thus, NCOAT is downstream of hormone and requires activation to reverse the actions of histone deacetylase and OGT. The aims of this project are as follows: 1. Further characterize transgene mice to determine the role of NCOAT in mammary development and other estrogen actions in the mammary gland. 2. Find where and how NCOAT is modified to activate it and what enzymes are involved. 3. Use the OGT interaction of NCOAT to design peptide therapy for breast cancer. This translational therapy will be developed to treat human breast cancer whose growth has become hormone-independent.